Question: This Northern hemisphere mystery bird's scientific name refers to one of its distinctive abilities (or perhaps I should instead say "poor abilities"). What is that?

Response: This is an adult common loon, Gavia immer, now more properly known by the compromise common name, the great northern loon. This individual is in winter plumage.

I apologize for my incorrectly-worded question. I actually meant to ask about the bird's common name, not its scientific name! Which means the response I prepared for you is wrong, because it doesn't answer the question that you all were puzzling over. I also cannot create an answer to the original (mis-worded) question because, well, there is no answer. So here's the question as it was supposed to have appeared, and the response I thought I was asking for.

Question: This Northern hemisphere mystery bird's American common name refers to one of its distinctive abilities (or perhaps I should instead say "poor abilities"). What is that?

Response: The North American name, "loon" is either derived from the Old English word lumme, meaning lummox or awkward person, or the Scandinavian word lum meaning lame or clumsy. Either way, the name refers to the loon's poor ability to walk on land. This is due to their body shape: since their webbed feet are located at the back of their bodies instead of underneath (as in ducks and geese), common loons are unable to walk or take-off from land. However, the placement of their legs and feet makes them powerful swimmers and divers, hence their common name in the EU: "diver".